Method for Quickly Identifying Disconnection of CT in Protection of 3/2 Connection Mode Based Bus

ABSTRACT

The present disclosure relates to a method for quickly identifying disconnection of a Current Transformer (CT) in protection of a 3/2 connection mode based bus. The present disclosure can quickly and accurately identify the disconnection of the CT, thereby avoiding the problem of a maloperation in protection of a bus caused by the disconnection of the CT.

This application claims the benefit of China Patent Application SerialNo. 201610002985.0, filed Jan. 4, 2016, which is hereby incorporated byreference in its entirety.

FIELD

The present disclosure relates to a method for quickly identifyingdisconnection of Current Transformer (CT) in protection of a 3/2connection mode based bus, and belongs to the technical field of ajudgment on a fault of a power system.

BACKGROUND

As the structure of a power grid is increasingly compact, stable runningof the power grid appears particularly important. For example, a 3/2connection mode is adopted for a bus of 1000 kV, 750 kV or 500 kV. Thereliability of the connection mode in supplying power is higher. Even ifa certain circuit breaker trips or a certain protection mal-operates,the reliability in supplying power will not be influenced. Accordingly,composite voltage locking is typically not configured for differentialprotection of 3/2 connection mode based bus protection. Moreover, thecomposite voltage locking serving as a maloperation measure of thedifferential protection cannot play a role in protection of the 3/2connection mode based bus.

In a newly-built substation, the number of strings of a 3/2 connectionmode bus is 2 to 4, and under most circumstances, the number is 2. Ifthere are only 2 strings, in the case where composite voltage locking isnot configured, after a branch CT is disconnected, a differentialcurrent caused by CT disconnection is equal to a load current of theother string. In such case, owing to the brake characteristic ofdifferential protection, it is difficult to brake the differentialprotection, thereby causing maloperation of the differential protectiondue to the CT disconnection. Therefore, since composite voltage lockingis not adopted for the differential protection of existing domestic 3/2connection mode based bus protection and there exists an upper limit ofthe number of times for reliable tripping of a high voltage circuitbreaker, how to prevent maloperation of 3/2 connection mode based busprotection during CT disconnection becomes a problem to be urgentlysolved.

SUMMARY

To overcome the defects in the conventional art, the present disclosureprovides a method for quickly identifying disconnection of a CT inprotection of 3/2 connection mode based bus, capable of solving thetechnical problem of a maloperation in protection of a high voltage buscaused by disconnection of a CT disconnection.

The present disclosure is implemented by the solutions as follows.

A method for quickly identifying disconnection of a CT in protection of3/2 connection mode based bus includes the following steps:

Step 1, collecting currents of all running branches on a current bus,calculating a current differential current and brake current, thenjudging, interval by interval, whether a load current abruptly varies,and if the load current of only one branch abruptly varies, targetingthe interval;

Step 2, judging whether a variation of the current differential currentin Step 1 is larger than a constant value of CT disconnection locking,and judging whether a variation of the current brake current meets afeature of decreasing;

Step 3, judging whether the targeted interval in Step 1 varies from astate in which there is the load current to a state in which there is nocurrent, and judging whether a variation in a current load current percycle of the targeted interval is equal to a variation in a currentdifferential current per cycle;

Step 4 if the above judgment conditions in Steps 2 and 3 are met,judging that the disconnection of the CT occurs at the targetedinterval.

Further, the differential current in Step 1 is a vector sum of thecurrents of all the branches, and the brake current is a scalar sum ofthe currents of all the branches.

Further, a criterion for judging whether the variation of the currentdifferential current is larger than the constant value of CTdisconnection locking in Step 2 is as follows:

|I _(cd(i)) −I _(cd(i−2T)) |>|I _(cd(i−2T)) −I _(cd(i−4T)) |+I_(CT disconnection locking constant value),

where I_(cd(i)) is a value of the current differential current,I_(cd(i−2T)) is a value of a differential current of a cycle which islocated before the current cycle and separated from the current cycle byone cycle, I_(cd(i−4T)) is a value of a differential current of a cyclewhich is located before the current cycle and separated from the currentcycle by three cycles, and I_(CT disconnection locking constant value)is the constant value of CT disconnection locking.

Further, a judgment condition for judging whether the variation of thecurrent brake current meets the feature of decreasing in Step 2 is asfollows:

|I _(r(i−2T)) −I _(r(i−4T)) |>|I _(r(i)) −I _(r(i−2T)) |+I _(mk2),

where I_(r(i)) is a value of the current differential current,I_(r(i−2T)) is a value of a differential current of a cycle which islocated before the current cycle and separated from the current cycle byone cycle, I_(r(i−4T)) is a value of a differential current of a cyclewhich is located before the current cycle and separated from the currentcycle by three cycles, and I_(mk2) is a fixed threshold value.

Further, a criterion for judging, interval by interval, whether thecurrent load current abruptly varies in Step 1 is as follows:

|I _(fh(i−2T)) −I _(fh(i−4T)) |>|I _(fh(i)) −I _(fh(i−2T)) |+I _(mk),

where I_(fh(i)) is a current load current, I_(fh(i−2T)) is a loadcurrent of a cycle which is located before the current cycle andseparated from the current cycle by one cycle, I_(fh(i−4T)) is a loadcurrent of a cycle which is located before the current cycle andseparated from the current cycle by three cycles, and I_(mk) is a fixedthreshold value.

Further, a judgment condition for judging whether the targeted intervalvaries from the state in which there is the load current to the state inwhich there is no current in Step 3 is as follows:

|I _(fh·A)|<0.04I _(n) or |I _(fh·B)|<0.04I _(n) or |I _(fh·C)|<0.04I_(n),

where I_(fh·A) is a value of an A-phase load current of the targetedinterval, I_(fh·B) is a value of an B-phase load current of the targetedinterval, I_(fh·C) is a value of an C-phase load current of the targetedinterval, and I_(n) is a current threshold value.

Further, a condition for judging whether the variation in the currentload current per cycle of the targeted interval is equal to thevariation in the current differential current per cycle in Step 3 is asfollows:

|I _(fh(i)) −I _(fh(i−2T)) |−|I _(cd(i)) −I _(cd(i−2T))|<0.06I _(n),

where I_(fh(i)) is a value of a current load current, I_(fh(i−2T)) is avalue of load current of a cycle which is located before the currentcycle and separated from the current cycle by one cycle, I_(cd(i)) is avalue of the current differential current, I_(cd(i−2T)) is a value of adifferential current of a cycle which is located before the currentcycle and separated from the current cycle by one cycle, and I_(n) is acurrent threshold value.

Compared with the conventional art, the present disclosure has thebeneficial effects as follows.

The present disclosure provides a method for quickly identifyingdisconnection of a CT in protection of a 3/2 connection mode based bus.In the present disclosure, a three-phase differential current andthree-phase brake current of each of the branches of a high voltage busare calculated, it is judged whether a load current abruptly variesinterval by interval, and the interval is targeted. Then, it issequentially judged whether the differential current, the brake currentand the load current at the targeted interval are 0 and whether avariation of the load current is equal to that of the differentialcurrent; if the judgment conditions are met, it is judged that adisconnection of the CT occurs at the interval. Due to of the fact thatcomposite voltage locking is not adopted for differential protection of3/2 connection mode based bus protection, the present disclosure canquickly and accurately identify the disconnection of the CT, therebyavoiding the problem of a maloperation in protection of a bus caused bythe disconnection of the CT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing a principle of an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure will be further described in detail below inconjunction with the drawings and the embodiment.

The present disclosure provides a method for quickly identifyingdisconnection of a CT in protection of a 3/2 connection mode based bus.FIG. 1 shows a flow of judgment, which includes the following specificsteps.

1. Sample values of three-phase differential currents and three-phasebrake currents of all running branches on a bus are calculated, whereineach three-phase differential current is a vector sum (Formula (1)) ofsampling currents of each branch, and each three-phase brake current isa scalar sum (Formula (2)) of the sampling currents of each branch:

Differential current:

$\begin{matrix}{i_{d} = {\sum\limits_{j = 1}^{n}i_{j}}} & (1)\end{matrix}$

Brake current:

$\begin{matrix}{i_{r} = {\sum\limits_{j = 1}^{n}{i_{j}}}} & (2)\end{matrix}$

2. It is judged whether a load current varies abruptly. Specifically, avariation of a value of a current sample of a current point with respectto a value of a current sample obtained in a cycle, which is locatedbefore the current cycle and separated from the current cycle by onecycle, is compared with a variation of the value of the current sampleobtained in the cycle, which is located before the current cycle andseparated from the current cycle by one cycle with respect to a value ofa current sample obtained in a cycle, which is located before thecurrent cycle and separated from the current cycle by three cycles. Ifthe differential current increases, it is indicated that a differencecurrent is generated. If the following criterion (Formula (2)) is met,it is proved that the currents of the branches abruptly vary. When thedifferential current of only one branch abruptly varies, a serial numberof the interval is targeted, and the following steps are performed. Thecriterion for judging whether the currents of the branches abruptly varyis as follows:

|I _(fh(i−2T)) −I _(fh(i−4T)) |>|I _(fh(i)) −I _(fh(i−2T)) |+I _(mk)  (3),

where I_(fh(i)) is a value of a load current of a current point,I_(fh(i−2T)) is a value of a load current of a cycle, which is locatedbefore the current cycle and separated from the current cycle by onecycle, of this point, I_(fh(i−4T)) is a load current of a cycle, whichis located before the current cycle and separated from the current cycleby three cycles, of this point, and I_(mk) is a fixed threshold value.The fixed threshold value is introduced in comparison of the twovariations for preventing that differential current is misjudged to beincreased due to a difference current generated from unbalancedcurrents.

3. It is judged whether a variation in a current differential currentper cycle is larger than a constant value of CT disconnection locking.That is, a variation of a value of a current sample of a differentialcurrent of a current point with respect to a value of a current sampleobtained in a cycle, which is located before the current cycle andseparated from the current cycle by one cycle, is compared with avariation of the value of the current sample obtained in a cycle, whichis located before the current cycle and separated from the current cycleby one cycle, with respect to a value of a current sample obtained in acycle, which is located before the current cycle and separated from thecurrent cycle by three cycles. A criterion is shown as Formula (4):

|I _(cd(i)) −I _(cd(i−2T)) |>|I _(cd(i−2T)) −I _(cd(i−4T)) |+I_(CT disconnection locking constant value)   (4),

where I_(cd(i)) is a value of a differential current of a current point,I_(cd(i−2T)) is a value of a differential current of a cycle, which islocated before the current cycle and separated from the current cycle byone cycle, of this point, I_(cd(i−4T)) is a value of a differentialcurrent of a cycle, which is located before the current cycle andseparated from the current cycle by three cycles, of this point, andI_(CT disconnection locking constant value) is a constant value of CTdisconnection locking. If the current of the current point meets thecriterion (Formula (4)), that is, an increased amplitude is larger thanthe constant value of CT disconnection locking, the next step isperformed.

4. It is judged whether a current brake current meets a feature ofdecreasing. That is, a variation of a value of a current sample of abrake current of a current point with respect to a value of a currentsample obtained in a cycle, which is located before the current cycleand separated from the current cycle by one cycle, is compared with avariation of the value of the current sample obtained in the cycle,which is located before the current cycle and separated from the currentcycle by one cycle, with respect to a value of a current sample obtainedin a cycle, which is located before the current cycle and separated fromthe current cycle by three cycles. A criterion is shown as Formula (5):

|I _(r(i−2T)) −I _(r(i−4T)) |>|I _(r(i)) −I _(r(i−2T)) |+I _(mk2)   (5),

where I_(r(i)) is a differential current value of a current point,I_(r(i−2T)) is a value of a differential current of a cycle, which islocated before the current cycle and separated from the current cycle byone cycle, of this point, I_(r(i−4T)) is a value of a differentialcurrent of a cycle, which is located before the current cycle andseparated from the current cycle by three cycles, of this point, andI_(mk2) is a fixed threshold value in Formula (5). The fixed thresholdvalue I_(mk2) is introduced in comparison of the two variations forpreventing the differential current is misjudged to be increased due toa difference current generated from unbalanced currents. If the currentof the current point meets the criterion (Formula (5)), the next step isperformed.

5. A load current at the targeted interval is calculated using afull-cycle Fourier algorithm for judging whether the targeted intervalvaries from a state in which there is the load current to a state inwhich there is no current. A criterion is as follows:

|I _(fh·A)|<0.04I _(n) or |I _(fh·B)|<0.04I _(n) or |I _(fh·C)|<0.04I_(n)   (6),

where I_(fh·A) is a value of an A-phase load current of a current point,I_(fh·B) is a value of a B-phase load current of the current point,I_(fh·C) is a value of a C-phase load current of the current point. Ifthe current of the current point meets the criterion (Formula (6)), thenext step is performed.

6. It is judged a feature where a variation of a load current is equalto a variation of a differential current in the targeted interval. Acriterion is as follows:

|I _(fh(i)) −I _(fh(i−2T)) |−|I _(cd(i)) −I _(cd(i−2T))|<0.06I _(n)  (7),

where I_(fh(i)) is a value of a load current of a current point,I_(fh(i−2T)) is a value of a load current of a cycle, which is locatedbefore the current cycle and separated from the current cycle by onecycle, of this point, I_(cd(i)) is a value of a differential current ofthe current point, I_(cd(i−2T)) is a value of a differential current ofa cycle, which is located before the current cycle and separated fromthe current cycle by one cycle, of this point, and I_(n) is a currentthreshold value. In order to eliminate influence caused by unbalancedcurrents, a non-current threshold. If the criterion (Formula (7)) ismet, it can be judged that the disconnection of the CT occurs at thetargeted interval, and corresponding measures are taken.

The number of sampling points in judgment in the present embodimentmatches sampling value differential protection. Generally, the samplingvalue differential protection is taking 6 from 12, so the number ofpoints meeting the criteria in Steps 1 and 4 may be set as 5. When fivesuccessive points meet the criteria, it is judged that a CTdisconnection criterion is pre-met, and in this case, differentialprotection action logics are forcibly strung into a time delay of 20 msto wait for judgments in Steps 5 and 6.

The present embodiment where two cycles serve as the period of a currentcycle is only a preferred embodiment, and is not intended to limit thepresent disclosure. As other implementations, other sampling values canbe selected to calculate variations of differential currents and brakecurrents, similarly.

Under an idea provided in the present disclosure, a technical way in theabove embodiment is converted, replaced and modified using a mode easilythought by those skilled in the art; moreover, achieved functions andpurposes are basically the same as those of corresponding technical waysin the present disclosure, a technical solution formed thereby is formedby slightly adjusting the above embodiment, and the technical solutionstill falls within the protective scope of the present disclosure.

What is claimed is:
 1. A method for quickly identifying disconnection of a Current Transformer (CT) in protection of 3/2 connection mode based bus, characterized by comprising: Step 1, collecting currents of all running branches on a current bus, calculating a current differential current and brake current, then judging, interval by interval, whether a load current abruptly varies, and if the load current of only one branch abruptly varies, targeting the interval; Step 2, judging whether a variation of the current differential current in Step 1 is larger than a constant value of CT disconnection locking, and judging whether a variation of the current brake current meets a feature of decreasing; Step 3, judging whether the targeted interval in Step 1 varies from a state in which there is the load current to a state in which there is no current, and judging whether a variation in a current load current per cycle of the targeted interval is equal to a variation in a current differential current per cycle; Step 4, if the above judgment conditions in Steps 2 and 3 are met, judging that the disconnection of the CT occurs at the targeted interval.
 2. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein the differential current in Step 1 is a vector sum of the currents of all the branches, and the brake current is a scalar sum of the currents of all the branches.
 3. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein a criterion for judging whether the variation of the current differential current is larger than the constant value of CT disconnection locking in Step 2 is as follows: |I _(cd(i)) −I _(cd(i−2T)) |>|I _(cd(i−2T)) −I _(cd(i−4T)) |+I _(constant value of CT disconnection locking), where I_(cd(i)) is a value of the current differential current, I_(cd(i−2T)) is a value of a differential current of a cycle which is located before the current cycle and separated from the current cycle by one cycle, I_(cd (i−4T)) is a value of a differential current of a cycle which is located before the current cycle and separated from the current cycle by three cycles, and I_(CT disconnection locking constant value) is the constant value of CT disconnection locking.
 4. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein a judgment condition for judging whether the variation of the current brake current meets the feature of decreasing in Step 2 is as follows: |I _(r(i−2T)) −I _(r(i−4T)) |>|I _(r(i)) −I _(r(i−2T)) |+I _(mk2), where I_(r(i)) is a value of the current brake current, I_(r(i−2T)) is a value of a brake current of a cycle which is located before the current cycle and separated from the current cycle by one cycle, I_(r(i−4T)) is a value of a brake current of a cycle which is located before the current cycle and separated from the current cycle by three cycles, and I_(mk2) is a fixed threshold value.
 5. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein a judgment condition for judging whether the variation of the current brake current meets the feature of decreasing in Step 2 is as follows: |I _(r(i−2T)) −I _(r(i−4T)) |>|I _(r(i)) −I _(r(i−2T)) |+I _(mk2), where I_(r(i)) is a value of the current brake current, I_(r(i−2T)) is a value of a brake current of a cycle which is located before the current cycle and separated from the current cycle by one cycle, I_(r(i−4T)) is a value of a brake current of a cycle which is located before the current cycle and separated from the current cycle by three cycles, and I_(mk2) is a fixed threshold value.
 6. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein a judgment condition for judging whether the targeted interval varies from the state in which there is the load current to the state in which there is no current in Step 3 is as follows: |I _(fh·A)|<0.04I _(n) or |I _(fh·B)|<0.04I _(n) or |I _(fh·C)|<0.04I _(n), where I_(fh·A) is a value of an A-phase load current of the targeted interval, I_(fh·B) is a value of an B-phase load current of the targeted interval, I_(fh·C) is a value of an C-phase load current of the targeted interval, and I_(n) is a current threshold value.
 7. The method for quickly identifying the disconnection of the CT in the protection of the 3/2 connection mode based bus of claim 1, wherein a condition for judging whether the variation in the current load current per cycle of the targeted interval is equal to the variation in the current differential current per cycle in Step 3 is as follows: |I _(fh(i)) −I _(fh(i−2T)) |−|I _(cd(i)) −I _(cd(i−2T))|<0.06I _(n), where I_(fh(i)) is a value of a current load current, I_(fh(i−2T)) is a value of a load current of a cycle which is located before the current cycle and separated from the current cycle by one cycle, I_(cd(i)) is a value of the current differential current, I_(cd(i−2T)) is a value of a differential current of a cycle which is located before the current cycle and separated from the current cycle by one cycle, and I_(n) is a current threshold value. 